1. Field
This invention relates to improvement in battery life (also called talk time or usage time) of mobile handsets for wireless communications systems.
2. Related Art
Time Division Multiplex Access (TDMA) based wireless communications systems such as Global System for Mobile communications (GSM), General Packet Radio Service (GPRS), Enhanced Data rates for GSM Evolution (EDGE) as well as future Time Division-Synchronous Code Division Multiple Access (TD-SCDMA) are used globally by 80% of mobile handset subscribers. Mobile handsets have become the platform to integrate possible applications like FM radio, MP3, camera, video, TV, Internet browsing, etc. While mobile handsets are continually adding new and innovative features and applications, handsets are battery-powered devices. The handset battery life will become increasing shorter and due to increasing power consumption budget.
Finding methods to increase the battery life is becoming a challenging issue for mobile handset designers. Analyzing power consumption budget in common handsets, it is found that up to 70% of total power is consumed by power amplifiers (PAs). Further analysis of handsets usage scenarios show handsets are operated mostly in its medium and low transmitted power levels. This knowledge leads to a methodology to increase handset talk time, improving power amplifier power efficiency for medium and low handset transmitting output power.
Conventional power amplifiers used in mobile handsets have low power efficiency at low transmitting power operation. A method of improving amplifier subsystem power efficiency at low transmitting power operation is the use of Doherty-type power amplifiers. Doherty-type amplifiers are designed to have high power efficiency over a wide output power range. The Doherty-type power amplifier subsystem will deliver significant improvement of power-added efficiency in medium and low transmitted power levels, which leads to longer battery life of handsets because of the handset usage scenarios. It also allows accurate power control, calibration and easy interface with base bands or RF transceivers.
The prior art focuses exclusively on the Doherty amplifier and the issues matching the Doherty amplifier output impedance to a fixed external impedance, as well as adjusting the Doherty amplifier bias to correct output power variation due to shifts in temperature or supply voltage. To implement the Doherty-type power amplifier subsystem for handset application several drawbacks need to be addressed. The first is the need to match the impedance seen by the Doherty-type amplifier from subsystem output (e.g. antenna, switch, diplexer). The second drawback to be overcome is to optimize the amplifier subsystem bias for maximum power efficiency as the transmitted output power is varying.